Hydrogenation of prochiral ketones

ABSTRACT

Process for enantioselectively hydrogenating prochiral ketones to (S)-alcohols using platinum catalysts in the presence of cinchonines or quinidines as modifiers and in the presence of hydrogen, which is characterized in that the modifiers used are cinchonines unsubstituted in the 3-position, 3-ethylidenyl- or 9-methoxycinchonines or derivatives thereof in which the quinoline ring is replaced by other rings.

The present invention relates to a process for enantioselectively hydrogenating prochiral ketones to (S)-alcohols using platinum catalysts in the presence of cinchonines or quinidines as modifiers and of hydrogen, which is characterized in that the modifiers used are cinchonines unsubstituted in the 3-position, 3-ethylidenyl- or 9-methoxycinchonines or derivatives thereof in which the quinoline ring is replaced by other rings.

The enantioselective hydrogenation of α-ketoesters using platinum catalysts in the presence of cinchonidine or cinchonine and derivatives of these quinuclidines has been described by H.-U. Blaser et al. in Catalysis Today 37 (1997), pages 441 to 463. This publication also discloses that the enantioselectivity in the presence of cinchonidine for preparing (R)-alcohols is considerably higher than in the presence of cinchonine for preparing (S)-alcohols. The same observation is made by B. Török et al. in Chem. Comm. (1999), pages 1725 to 1726 in the enantioselective hydrogenation of an α-ketodiacetal. The hydrogenation of α-ketoacetals is also described by M. Studer et al. in Chem. Comm. (1999), pages 1727 to 1728. In J. Am. Chem. Soc. (2000) 122, pages 12675 to 12682, H. U. Blaser describes the influence of modification of cinchona alkaloids on the hydrogenation of ethyl pyruvate using cinchona-modified platinum catalysts. It is established that the substitution in the 3-position of the quinuclidine radical has virtually no or only a small influence. In connection with the determination of the pK_(a) values of cinchona alkaloids, C. Drzewiczak et al. in Polish J. Che., 67, 48ff (1993) mention 3-ethylidenecinchonine without specifying a synthesis or use.

It has now been found that, surprisingly, it is possible to achieve a distinctly higher catalyst activity and increased enantioselectivity in the hydrogenation of prochiral ketones to (S)-alcohols using hydrogen when platinum catalysts are modified with 3-ethylidene- or 9-methoxycinchonines or derivatives thereof in which the quinoline ring is replaced by other rings. The optical yields of (S)-alcohols may be over 90% ee and such high yields could hitherto be achieved in the preparation of (S)-alcohols by this hydrogenation route only by the use of ultrasound (B. Török et al., Ultrasonics Sonochemistry 7 (2000) 151) or by continuously adding modifier (C. LeBlond et al., JACS 121 (1999) 4920).

The invention provides a process for enantioselectively hydrogenating prochiral ketones to (S)-alcohols using platinum catalysts in the presence of cinchonines or quinidines as modifiers and in the presence of hydrogen, which is characterized in that the modifiers used are cinchonines from the group of cinchonines unsubstituted in the 3-position, 3-ethylidenyl- or 9-methoxycinchonines or derivatives thereof in which the quinoline ring is replaced by other rings.

Prochiral ketones are well known. The prochiral α-ketones may be saturated or unsaturated, open-chain or cyclic compounds which preferably have 5 to 30, more preferably 5 to 20, carbon atoms which are unsubstituted or substituted with radicals which are stable under the hydrogenation conditions. The carbon chain may be interrupted by heteroatoms, preferably from the group of —O—, ═N— and —NR′—, where R′ is H, C₁-C₈-alkyl, preferably C₁-C₄-alkyl, C₅-C₈-cycloalkyl, for example cyclopentyl, cyclohexyl or cyclooctyl, C₆-C₁₀-aryl, for example phenyl or naphthyl, or C₇-C₁₂-aralkyl, for example phenylmethyl or phenylethyl. The prochiral ketones preferably have an activating group in the α-position, for example a carboxyl, carboxylic ester, acetal, keto or ether group.

The prochiral ketones may be α-ketocarboxylic acids, α-ketocarboxylic esters, α-ketoethers, α-ketoacetals and α,β-diketones. These prochiral ketones may correspond to the formulae I, II, III, IV and V

where

R₁, R₂, R₃ and R₆ are each independently a monovalent, saturated or unsaturated aliphatic radical having 1 to 12 carbon atoms, a saturated or unsaturated cycloaliphatic radical having 3 to 8 carbon atoms, a saturated or unsaturated heterocycloaliphatic radical having 3 to 8 ring members and one or two heteroatoms from the group of O, N and NR′, a saturated or unsaturated cycloaliphatic-aliphatic radical having 4 to 12 carbon atoms, a saturated or unsaturated heterocycloaliphatic-aliphatic radical having 3 to 12 carbon atoms and one or two heteroatoms from the group of O, N and NR′, an aromatic radical having 6 to 10 carbon atoms, a heteroaromatic radical having 4 to 9 carbon atoms and one or two heteroatoms from the group of O and N, an aromatic-aliphatic radical having 7 to 12 carbon atoms or a heteroaromatic-aliphatic radical having 5 to 11 carbon atoms and one or two heteroatoms from the group of O and N where R′ is H, C₁-C₈-alkyl, preferably C₁-C₄-alkyl, C₅- or C₆-cycloalkyl, C₆-C₁₀-aryl, for example phenyl or naphthyl, C₇-C₁₂-aryl, for example phenylmethyl or phenylethyl,

R₁ and R₂ or R₁ and R₆ together are C₁-C₆-alkylene or C₃-C₈-1,2-cycloalkylene, or C₂-C₄-alkylene or C₃-C₈-cycloalkylene fused to 1,2-phenylene, and R₃ is as defined above,

R₂ and R₃ together are C₁-C₆-alkylene, C₁-C₈-alkylidene, C₃-C₈-1,2-cycloalkylene, C₃-C₈-cycloalkylidene, benzylidene, 1,2-phenylene, 1,2-pyridinylene, 1,2-naphthylene, or C₃-C₄-alkylene or C₃-C₈-1,2-cycloalkylene fused to 1,2-cycloalkylene or 1,2-phenylene, and R₁ is as defined above,

and R₁, R₂, R₃ and R₆ are each unsubstituted or substituted by one or more, identical or different radicals selected from the group of C₁-C₄-alkyl, C₂-C₄-alkenyl, C₁-C₄-alkoxy, C₁-C₄-haloalkyl, C₁-C₄-hydroxyalkyl, C₁-C₄-alkoxymethyl or -ethyl, C₁-C₄-haloalkoxy, cyclohexyl, cyclohexyloxy, cyclohexylmethyl, cyclohexylmethyloxy, phenyl, phenyloxy, benzyl, benzyloxy, phenylethyl, phenylethyloxy, halogen, —OH, —OR₄, —OC(O)R₄, —NH₂, —NHR₄, —NR₄R₅, —NH—C(O)—R₄, —NR₄—C(O)—R₄, —CO₂R₄, —CO₂—NH₂, —CO₂—NHR₄, —CO₂—NR₄R₅ where R₄ and R₅ are each independently C₁-C₄-alkyl, cyclohexyl, cyclohexylmethyl, phenyl or benzyl.

The heterocyclic radicals are bonded via a ring carbon atom to the oxygen atoms or the carbon atom of the carbonyl groups in the compounds of the formulae I, II, III, IV and V.

Preferred substituents are methyl, ethyl, n- and i-propyl, n- and t-butyl, vinyl, allyl, methyloxy, ethyloxy, n- and i-propyloxy, n- and t-butyloxy, trifluoromethyl, trichloromethyl, β-hydroxyethyl, methoxy- or ethoxymethyl or -ethyl, trifluoromethoxy, cyclohexyl, cyclohexyloxy, cyclohexylmethyl, cyclohexylmethyloxy, phenyl, phenyloxy, benzyl, benzyloxy, phenylethyloxy, phenylethyl, halogen, —OH, —OR₄, —OC(O)R₄, —NH₂, —NHR₄, —NR₄R₅, —NH—C(O)—R₄, —NR₄—C(O)—R₄, —CO₂R₄, —CO₂—NH₂, —CO₂—NHR₄, —CO₂—NR₄R₅ where R₄ and R₅ are each independently C₁-C₄-alkyl, cyclohexyl, cyclohexylmethyl, phenyl or benzyl.

The aliphatic radical is preferably alkyl which may be linear or branched and preferably has 1 to 8, more preferably 1 to 4, carbon atoms, or preferably alkenyl or alkynyl, each of which may be linear or branched and preferably have 2 to 8, more preferably 2 to 4, carbon atoms. When R₂ and R₃ are alkenyl or alkynyl, the unsaturated bond is preferably in the β-position to the oxygen atom. Examples include methyl, ethyl, n- and i-propyl, n-, i- and t-butyl, pentyl, i-pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl and dodecyl, vinyl, allyl, ethynyl and propargyl. A preferred group of aliphatic radicals is methyl, ethyl, n- and i-propyl, n-, i- and t-butyl.

The cycloaliphatic radical is preferably cycloalkyl or cycloalkenyl having preferably 3 to 8, more preferably 5 or 6, ring carbon atoms. Some examples are cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl and cyclooctyl, and also cyclopentenyl, cyclohexenyl and cyclohexadienyl. Particular preference is given to cyclopentyl and cyclohexyl.

The heterocycloaliphatic radical is preferably heterocycloalkyl or heterocycloalkenyl having preferably 3 to 6 carbon atoms, 4 to 7 ring members and heteroatoms selected from the group of —O— and —NR′— where R′ is H, C₁-C₈-alkyl, preferably C₁-C₄-alkyl, C₅- or C₆-cycloalkyl, C₆-C₁₀-aryl, for example phenyl or naphthyl, phenyl or phenylethyl. Some examples are pyrrolidinyl, pyrrolinyl, tetrahydrofuranyl, dihydrofuranyl and piperazinyl.

The cycloaliphatic-aliphatic radical is preferably cycloalkyl-alkyl or -alkenyl having preferably 3 to 8, more preferably 5 or 6, ring carbon atoms, and preferably 1 to 4, or 2-4, more preferably 1 or 2, or 2 or 3, carbon atoms in the alkyl group and alkenyl groups respectively. Examples include cyclopentyl- or cyclohexylmethyl or -ethyl and cyclopentyl- or cyclohexylethenyl.

The heterocycloaliphatic-aliphatic radical is preferably heterocycloalkyl-alkyl or -alkenyl having preferably 3 to 6 carbon atoms, 4 to 7 ring members and heteroatoms selected from the group of —O— and —NR′— where R′ is H, C₁-C₈-alkyl, preferably C₁-C₄-alkyl, C₅- or C₆-cloalkyl, C₆-C₁₀-aryl, for example phenyl or naphthyl, phenyl or phenylethyl, and preferably 1 to 4, more preferably 1 or 2, carbon atoms in the alkyl group and 2 to 4, more preferably 2 or 3, carbon atoms in the alkenyl group. Examples include pyrrolidinylmethyl or -ethyl or -ethenyl, pyrrolinylmethyl or -ethyl or -ethenyl, tetrahydrofuranylmethyl or -ethyl or -ethenyl, dihydrofuranylmethyl or -ethyl or -ethenyl, and piperazinylmethyl or -ethyl or -ethenyl.

The aromatic radicals are preferably naphthyl and in particular phenyl.

The aromatic-aliphatic radicals are preferably phenyl- or naphthyl-C₁-C₄-alkyl or -C₂-C₄-alkenyl. Some examples are benzyl, naphthylmethyl, β-phenylethyl and β-phenylethenyl.

The heteroaromatic radicals are preferably 5- or 6-membered, optionally fused ring systems. Some examples are pyridinyl, pyrimidinyl, pyrazinyl, pyrrolyl, furanyl, oxazolyl, imidazolyl, benzofuranyl, indolyl, benzimidazolyl, quinolinyl, isoquinolinyl, quinazolinyl, quinoxalinyl.

The heteroaromatic-aliphatic radicals are preferably 5- or 6-membered, optionally fused ring systems which are bonded via one of their carbon atoms to the free bond of an alkyl group or alkenyl group where the alkyl group preferably contains 1 to 4, more preferably 1 or 2, carbon atoms, and the alkenyl group preferably contains 2 to 4, more preferably 2 or 3, carbon atoms. Some examples are pyridinylmethyl or ethyl or -ethenyl, pyrimidinylmethyl or -ethyl or -ethenyl, pyrrolylmethyl or -ethyl or -ethenyl, furanylmethyl or -ethyl or -ethenyl, imidazolylmethyl or -ethyl or -ethenyl, indolylmethyl or -ethyl or -ethenyl.

R₆ is preferably an aliphatic, cycloaliphatic or araliphatic radical, and more preferably linear C₁-C₄-alkyl.

More preferred compounds of the formulae I, II, III, IV and V include those where

R₁, R₂, R₃ and R₆ are each independently linear or branched C₁-C₈-alkyl, C₄-C₇-cycloalkyl or C₄-C₆-heterocycloalkyl having heteroatoms from the group of O and N, C₆-C₁₀-aryl or C₄-C₉-heteroaryl having heteroatoms from the group of O and N, C₄-C₇-cycloalkyl-C₁-C₄-alkyl or C₃-C₆-heterocycloalkyl-C₁-C₄-alkyl having heteroatoms from the group of O and N, C₆-C₁₀-aryl-C₁-C₄-alkyl or C₄-C₉-heteroaryl-C₁-C₄-alkyl having heteroatoms from the group of O and N,

R₁ and R₂ or R₁ and R₆ together are C₁-C₄-alkylene or C₄-C₇-1,2-cycloalkylene, or C₂-C₄-alkylene or C₄-C₇-cycloalkylene fused to 1,2-phenylene, and R₃ is as defined above,

R₂ and R₃ together are C₁-C₄-alkylene, C₁-C₄-alkylidene, C₄-C₇-1,2-cycloalkylene, C₄-C₇-cycloalkylidene, benzylidene, 1,2-phenylene, 1,2-pyridinylene, 1,2-naphthylene, or C₃-C₄-alkylene or C₄-C₇-cloalkylene fused to 1,2-cycloalkylene or 1,2-phenylene, and R₁ is as defined above

where R₁, R₂, R₃ and R₆ are each unsubstituted or substituted by one or more, identical or different radicals selected from the group of C₁-C₄-alkyl, C₁-C₄-alkoxy, C₁-C₄-haloalkyl, C₁-C₄-hydroxyalkyl, C₁-C₄-alkoxymethyl or -ethyl, C₁-C₄-haloalkoxy, cyclohexyl, cyclohexyloxy, cyclohexylmethyl, cyclohexylmethyloxy, phenyl, phenyloxy, benzyl, benzyloxy, phenylethyl, phenylethyloxy, halogen, —OH, —OR₄, —OC(O)R₄, —NH₂, —NHR₄, —NR₄R₅, —NH—C(O)—R₄, —NR₄—C(O)R₄, —CO₂R₄, —CO₂—NH₂, —CO₂—NHR₄, —CO₂—NR₄R₅ where R₄ and R₅ are each independently C₁-C₄-alkyl, cyclohexyl, phenyl or benzyl.

A preferred subgroup of the compounds of the formulae I, II, III, IV and V are those where

R₁, R₂, R₃ and R₆ are each independently linear or branched C₁-C₄-alkyl, C₂-C₄-alkenyl, C₅-C₆-cycloalkyl, phenyl, phenylethenyl, C₅-C₆-cycloalkyl-C₁-C₂-alkyl, or C₆-C₁₀-aryl-C₁-C₂-alkyl,

R₁ and R₂ or R₁ and R₆ together are C₁-C₃-alkylene or C₅-C₆-1,2-cycloalkylene,

R₂ and R₃ together are C₂-C₄-alkylene, C₁-C₄-alkylidene, C₅-C₆-1,2-cycloalkylene, C₅-C₆-cycloalkylidene, benzylidene, 1,2-phenylene

where R₁, R₂, R₃ and R₆ are each unsubstituted or substituted as defined previously.

A particularly preferred subgroup of the compounds of the formulae I, II, III, IV and V are those where

R₁ and R₆ are each C₁-C₄-alkyl, C₂-C₄-alkenyl, cyclohexyl, phenyl, benzyl, phenylethyl or phenylethenyl,

R₂ and R₃ are each independently linear or branched C₁-C₄-alkyl, cyclohexyl, phenyl, benzyl or phenylethyl,

R₁ and R₂ or R₁ and R₆ together are C₂-C₃-alkylene or 1,2-cyclohexylene,

R₂ and R₃ together are C₂-C₃-alkylene, C₁-C₄-alkylidene, 1,2-cyclohexylene, cyclohexylidene, benzylidene or 1,2-phenylene

where R₁, R₂, R₃ and R₆ are each unsubstituted or substituted by methyl, ethyl, n- and i-propyl, n- and t-butyl, vinyl, allyl, methyloxy, ethyloxy, n- and i-propyloxy, n- and t-butyloxy, trifluoromethyl, trichloromethyl, β-hydroxyethyl, methoxy- or ethoxymethyl or -ethyl, trifluoromethoxy, cyclohexyl, cyclohexyloxy, cyclohexylmethyl, cyclohexylmethyloxy, phenyl, phenyloxy, benzyl, benzyloxy, phenylethyloxy, phenylethyl, halogen, —OH, —OR₄, —OC(O)R₄, —NH₂, —NHR₄, —NR₄R₅, —NH—C(O)—R₄, —NR₄—C(O)—R₄, —CO₂R₄, —CO₂—NH₂, —CO₂—NH₄, —CO₂—NR₄R₅ where R₄ and R₅ are each independently C₁-C₄-alkyl, cyclohexyl, cyclohexylmethyl, phenyl or benzyl.

Some of the compounds of the formulae I, II, III, IV and V are known or can be prepared in a manner known per se by means of similar processes.

The compounds of the formulae I, II, III, IV and V are hydrogenated to chiral secondary alcohols of the formulae VI, VII, VIII and IX

where R₁, R₂, R₃ and R₆ are each as previously defined and the symbol * represents predominantly the S-form of one of the stereoisomers.

Platinum catalysts are known, extensively described and commercially available. It is possible to use either platinum in metal form, for example as a powder, or, which is preferred, platinum metal applied to finely divided supports. Examples of suitable supports include carbon, metal oxides, for example SiO₂, TiO₂, Al₂O₃, metal salts, and natural or synthetic silicates. The catalyst may also be a platinum colloid. The amount of platinum metal on the support may be, for example, 1 to 10% by weight, preferably 3 to 8% by weight, based on the support. Before their use, the catalysts may be activated by treating with hydrogen at elevated temperature and/or with ultrasound. Preferred catalysts are platinum on Al₂O₃.

The cinchonines unsubstituted in the 3-position, 3-ethylidenyl- or 9-methoxycinchonines or derivatives thereof to be used according to the invention may, for example, correspond to the formula XI with 8(R),9(S)-configuration

where

R₉ is CH₂═CH— or CH₃CH₂— and R₇ is methyl, or

R₉ is H or CH₃—CH═ and R₇ is H or methyl, and

R₈ is unsubstituted or C₁-C₄-alkyl- or C₁-C₄-alkoxy-substituted C₆-C₁₄-aryl or C₅-C₁₃-heteroaryl having heteroatoms selected from the group of —N═, —O—, —S— and —N(C₁-C₄-alkyl)-.

R₈ as aryl and heteroaryl may be a monocyclic or fused polycyclic radical having preferably 2 or three rings. The rings preferably contain 5 or 6 ring members. Some examples are phenyl, furyl, thiophenyl, N-methylpyrrolyl, pyridinyl, naphthyl, tetrahydronaphthyl, anthracenyl, phenanthryl, quinolinyl, tetrahydroquinolinyl, isoquinolinyl, indenyl, fluorenyl, benzofuranyl, dihydrobenzofuranyl, benzothiophenyl, dihydrobenzothiophenyl, N-methylindolyl, dihydro-N-methylindolyl, dibenzofuranyl, dibenzothiophenyl and N-methylcarbazolyl.

The cinchonines unsubstituted in the 3-position, 3-ethylidenyl- or 9-methoxycinchonines or derivatives thereof to be used according to the invention preferably correspond to the formula XIa with 8(R),9(S)-configuration

where

R₉ is CH₂═CH— or CH₃CH₂— and R₇ is methyl, or

R₉ is H or CH₃—CH═ and R₇ is H or methyl,

R₈ is a radical of the formulae

and R₁₀ is H, OH or C₁-C₄-alkoxy.

R₁₀ is preferably H, OH or methoxy.

The compounds of the formula XI where R₉ is CH₂═CH— or CH₃CH₂— and R₇ is methyl may be prepared in a simple manner by methylating the hydroxyl group bonded to C9 of appropriate natural cinchona alkaloids. Compounds where R₉ is ethyl are obtainable by hydrogenating the R₉ vinyl group.

The compounds of the formula XI where R₉ is CH₃—CH═ may be prepared by isomerizing the R₉ vinyl group in the presence of metal complexes, for example ruthenium/phosphine complexes. An implementation of the process is described in the examples. In general, mixtures of the Z- and E-isomers are obtained which can be used directly as such.

The compounds of the formula XI which are not derived from natural cinchonines are synthetically accessible, for example, by means of reacting quinuclidine N-oxide with lithium alkyls (Li-methyl or Li-n-butyl) with aldehydes R₈—CH═O, subsequent reaction with a Lewis acid, for example TiCl₃, and ensuing alkaline hydrolysis. The diastereomers may be separated chromatographically on silica gel, and the enantiomers may be separated chromatographically on chiral columns. This is described in more detail in the examples.

The platinum metal may be used, for example, in an amount of 0.01 to 10% by weight, preferably 0.05 to 10% by weight and more preferably 0.1 to 5% by weight, based on the prochiral ketone used, although amounts of 0.1 to 3% by weight, or 0.1 to 1% by weight generally suffice. The increased activity of the hydrogenation system to be used according to the invention allows smaller total amounts of catalyst, which makes the process more economic.

The modifier may be used, for example, in an amount of 0.1 to 10 000% by weight, preferably 1 to 500% by weight and more preferably 10 to 200% by weight, based on the platinum metal used. The modifier may be introduced into the reaction vessel together with the platinum metal catalyst, or the platinum metal catalyst may be impregnated beforehand with the modifier.

The hydrogenation is preferably carried out under a hydrogen pressure of up 200 bar, more preferably up to 150 bar and particularly preferably 10 to 100 bar.

The reaction temperature may be, for example, −50 to 100° C., more preferably 0 to 50° C. and particularly preferably 0 to 35° C. It is generally possible to achieve better enantioselectivies at low temperatures.

The reaction may be carried out without or in an inert solvent or mixtures of solvents. Examples of suitable solvents include aliphatic, cycloaliphatic and aromatic hydrocarbons (pentane, hexane, petroleum ether, cyclohexane, methylcyclohexane, benzene, toluene, xylene), ethers (diethyl ether, dibutyl ether, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, tetrahydrofuran, dioxane), water, alcohols (methanol, ethanol, propanol, butanol, ethylene glycol, diethylene glycol, ethylene glycol monomethyl or monoethyl ether, diethylene glycol monomethyl or monoethyl ether), ketones (acetone, methyl isobutyl ketone), carboxylic esters and lactones (ethyl or methyl acetate, valerolactone), N-substituted carboxamides and lactams (dimethylformamide, N-methylpyrrolidone), and carboxylic acids (acetic acid, propionic acid, butyric acid). The choice of the solvent may be used to influence the optical yield. For example, aromatic hydrocarbons (benzene, toluene, xylene) have proven particularly useful in the case of α-ketoacetals and aromatic α-ketocarboxylic esters, while better results can be achieved using carboxylic acids, for example acetic acid, in the case of aliphatic α-ketocarboxylic acids.

The process according to the invention may, for example, be carried out in such a way that the catalyst is initially charged in an autoclave with the nitrogen base, optionally with a solvent, then the prochiral α-ketone is added, then the air is displaced with an inert gas, for example noble gases, hydrogen is injected in and then the reaction is started, optionally with stirring or shaking, and hydrogenation is effected until no more hydrogen takeup is observed. The α-hydroxyl compound formed may be isolated and purified by customary methods, for example distillation, crystallization and chromatographic methods.

The invention also provides compounds of the formula XIb

where

R₉ is CH₂═CH— or CH₃CH₂— and R₇ is methyl, or

R₉ is H or CH₃—CH═ and R₇ is H or methyl, and

R₁₀ is H or C₁-C₄-alkoxy.

When R₇ is H, R₁₀=H and R₉ is CH₂═CH—, the molecule is cinchonine (Cn) and when R₇ is H, R₁₀=H and R₉ is CH₃CH₂—, the molecule is hydrocinchonine (HCn).

The (S)-α-alcohols which can be prepared according to the invention are valuable intermediates for the preparation of natural active ingredients (B. T. Cho et al. in Tetrahedron: Asymmetry Vol. 5, No. 7 (1994), pages 1147 to 1150), and synthetic active pharmaceutical ingredients and pesticides. The (S)-α-alcohols obtainable may be converted beforehand by known processes to derivatives which may then be used as intermediates for the preparation of active ingredients. The acid hydrolysis of, for example, α-ketoacetals leads to 1,4-dioxanes or the corresponding aldehydes which are either hydrogenated to 1,2-diols having a secondary optically active hydroxyl group, or reacted with amines in the presence of phenylboric acids to optionally substituted optically active 1-phenyl-1-amino-2-hydroxyalkanes. After the protection of the OH group, for example by reaction with benzyl bromide, the hydroxyl-protected aldehydes may be obtained by reacting with strong acids and may be hydrogenated to 1,2-diols or converted to S-α-hydroxycarboxylic acids by oxidation (for example with chromium trioxide) and removing the protecting group.

The examples which follow illustrate the invention in detail. The optical yield is determined by gas chromatography using a Supelco Beta-dex column (article No. 2-4301), hydrogen as the carrier gas and elevated temperatures; or by means of HPLC (Chiracel OD column, using 95:5 hexane/isopropanol). The conversion is determined by means of ¹H NMR.

A) Preparation of Modifiers EXAMPLE A1 Preparation of O-Methylcinchonine (MeO-Cn, R₇ in Formula XIb=Methyl, R₁₀=H, R₉=CH₂═CH—)

0.60 g of potassium hydride (15.0 mmol) is weighed into a 250 ml two-necked flask equipped with a reflux condenser and dropping funnel under argon. This is washed three times with absolute n-pentane and subsequently suspended in 50 ml of absolute tetrahydrofuran. 3.24 g (11.0 mmol) of cinchonine (Cn) are then added in portions with ice cooling, and obvious gas development can be observed. After completion of addition, stirring is continued at 0° C. for about another half hour until an almost clear orange solution is obtained. The solution is then heated to 50° C. for a further 2 hours until no more gas development can be detected. At room temperature (RT), 0.69 ml (1.56 g; 11.0 mmol) of iodomethane are then slowly added dropwise. The solution is stirred at RT for 12 hours and then hydrolyzed using 50 ml of H₂O with ice cooling. The organic and the aqueous phases are separated, and the aqueous phase is extracted three times more with ethyl acetate (EA). The combined organic phases are dried over MgSO₄ and concentrated on a rotary evaporator (RE). Chromatographic purification on a silica gel column (EA/Nethyl₃ 9:1) and drying under high vacuum give 2.82 g (83%) of the title compound as a pale yellow solid. Recrystallization from a little n-hexane provides 2.50 g (74%) of colourless, rhombic crystals. Melting point: 113-114° C.; [α]_(D) ²⁰: +242° (c=0.90, CHCl₃).

EXAMPLE A2 Preparation of O-Methylquinidine (MeO-Qd, R₇ in Formula XIb=Methyl, R₁₀=Omethyl, R₉=CH₂═CH—)

The procedure of Example A1 is followed using quinidine. The title compound is obtained in a yield of 71% as a yellow, viscous oil. [α]_(D) ²⁰: +202° (c=0.78, CHCl₃).

EXAMPLE A3 Preparation of (E)/(Z)-Isocinchonine (iso-Cn, R₇ in Formula XIb=H, R₁₀=H, R₉=CH₃—CH═)

In a 100 ml two-necked flask equipped with a reflux condenser, 106.0 mg (408 μmol) of triphenylphosphine and 25.0 mg (100 μmol) of RuCl₃.nH₂O in 30 ml absolute dimethylformamide under argon are heated to 150° C. until a dear orange solution is formed (approx. 15 minutes). The solution is subsequently allowed to cool to 100° C., then 2 g (6.8 mmol) of cinchonine are added and the solution is heated once again to 150° C. for half an hour. The still-hot reaction mixture is poured into 100 ml of precooled water and stirred at 5° C. for 1 hour. The precipitated colourless solid is filtered off and dried under high vacuum. After recrystallization from dimethoxyethane, 1.10 g (55%) of the title compound as an inseparable 1:1 mixture of the Z- and E-isomers are obtained in the form of fine, colourless needles. Melting point of the diastereomer mixture: 229-231° C.; [α]_(D) ²⁰: +173° (c=0.93, CHCl₃).

EXAMPLE A4 Preparation of (E)/(Z)-Apoisoquinidine (iso-Qd, R₇ in the Formula XIb=H, R₁₀=Omethyl, R₉=CH₃—CH═)

The procedure of Example A3 is followed using quinidine. For isolation, the reaction mixture after aqueous workup is initially adjusted to a pH of 9-10 using 1 M NaOH solution and then extracted repeatedly with methylene chloride. The combined organic phases are concentrated under high vacuum and then the residue is recrystallized from diethyl ether. The title compound is obtained as an inseparable 1:1 mixture of the Z- and E-isomers in the form of a beige solid. Melting point of the diastereomer mixture: 161-165° C.; [α]_(D) ²⁰: +148° (c=0.88, CHCl₃).

EXAMPLE A5 Preparation of Rubanol, R₇ in Formula XIa=H, R₉=H, R₈=Naphthyl

BuLi: n-butyllithium; TMEDA: tetramethylethylenediamine.

3.58 ml (5.7 mmol) of n-butyllithium (1.6 M in n-hexane) are added dropwise at −78° C. within 30 min to a solution of 0.66 g (5.2 mmol) of azabicyclo[2.2.2]octane N-oxide and 0.86 ml (0.67 g; 5.7 mmol) of TMEDA in 30 ml of absolute THF. The yellow reaction solution is stirred at −78° C. for 1 h. 0.78 g (5.0 mmol) of α-naphthaldehyde in 10 ml of absolute tetrahydrofuran is then added slowly. Stirring is continued at −78° C. for 2 h and the mixture is then subsequently heated within 12 h to room temperature (RT). After adding 10 ml of saturated, aqueous NH₄Cl solution, the mixture is stirred at RT for 30 minutes.

The reduction of the N-oxide to the tertiary amine is carried out in situ using TiCl₃ solution (1.9 M in 2.0 M aqueous HCl) without further workup. Titanium(III) hydrochloric acid solution is added with ice cooling until a deep violet colour remains even after prolonged stirring. After heating to RT, the reaction mixture is set to pH=10 using 15 per cent aqueous NaOH solution. The precipitated salts are filtered through Celite, and the filtrate is repeatedly extracted using ethyl acetate. The combined organic phases are washed with saturated aqueous NaCl solution, dried over potassium carbonate, filtered and concentrated on a rotary evaporator. The ¹H NMR spectrum of the crude product shows that the two diastereomers are formed in a 1:1 ratio. Chromatographic purification on a silica gel column (ethyl acetate/triethylamine, 9:1) provides 0.55 g (41%) of the desired erythro-isomer as colourless needles. Preparative HPLC (Daicel Chiralcel OD®, 20×250 mm, n-hexane/isopropanol, 95:5, 1% of diethylamine), 20.0 ml/min, t_(r)[(−)-enantiomer]=10.4 min, t_(r)[(+)-enantiomer]=15.9 min) separates the two erythro-enantiomers to 98% ee in each case. ¹H NMR (CDCl₃, 400 MHz): 8.06 (d, 1H, ³J=8.2 Hz), 7.86 (dd, 1H, ³J=7.8 Hz, ⁴J=1.5 Hz), 7.76 (d, 1H, ³J=8.2 Hz), 7.71 (d, 1H, ³J=7.1 Hz), 7.48-7.41 (m, 3H), 5.77 (d, 1H, ³J=4.6 Hz), 3.58-2.52 (m, 6H), 1.91-1.31 (m, 7H). ¹³C NMR (CDCl₃, 101 MHz): 139.7 (q), 133.8 (q), 130.5 (q), 128.9 (t), 127.8 (t), 126.0 (t), 125.4 (2x t), 123.4 (t), 123.2 (t), 72.9 (t), 60.1 (t), 50.7 (s), 43.9 (s), 26.6 (s), 26.5 (s), 25.8 (s), 22.1 (t).

Melting point: 202-204° C. [α]_(D) ²⁰: −135° (c=0.35, CHCl₃).

EXAMPLE A6 Preparation of EXN-1, R₇ in Formula XIa=H, R₉=H, R₈=Quinoline

The synthesis is carried out in a similar manner to Example A5 using 3.58 g (28.2 mmol) of azabicyclo[2.2.2]octane N-oxide, 4.70 ml (31.1 mmol) of TMEDA, 20.00 ml (32.0 mmol) of n-butyllithium (1.6 M in n-hexane) and 5.00 g (31.7 mmol) of quinoline-4-carbaldehyde. The ¹H NMR spectrum of the crude product shows that the two diastereomers are formed in a 1:1 ratio. Chromatographic purification on a silica gel column (ethyl acetate/triethylamine, 9:1) provides 2.95 g (39%) of rubanol as a colourless solid. Semipreparative HPLC (Chiracel OD-H®, n-heptane/isopropanol 98:2, 0.5 ml/min, t_(r)[(−)-rubanol]=52.1 min, t_(r)[(+)-rubanol]=63.8 min) separates the erythro-enantiomers from each other to 99% ee in each case. ¹H NMR (CDCl₃, 400 MHz): 8.90 (d, 1H, ³J=4.6 Hz), 8.12 (dd, 1H, ³J=8.6 Hz, ⁴J=0.8 Hz), 7.97 (d, 1H, ³J=8.4 Hz), 7.69-7.64 (m, 2H), 7.43 (dt, 1H, ³J=7.0 Hz, ⁴J=1.2 Hz), 5.78 (d, 1H, ³J=3.5 Hz), 4.70 (br, 1H), 3.57-3.52 (m, 1H), 3.14-2.49 (m, 4H), 1.89-1.27 (m, 7H). ¹³C NMR (CDCl₃, 101 MHz): 150.6 (q), 148.7 (t), 148.4 (q), 130.8 (t), 129.4 (t), 126.9 (t), 126.1 (t), 125.5 (q), 123.4 (t), 118.7 (t), 72.0 (t), 60.4 (t), 51.1 (s), 44.3 (s), 26.7 (s), 26.3 (s), 25.9 (s), 22.3 (t). Melting point: 222-224° C. [α]_(D) ²⁰: +99° (c=0.51, CHCl₃).

B) Hydrogenations of Prochiral α-Ketones EXAMPLES B1-B8 Hydrogenation of Methyl Pyruvate [CH₃—C(O)—COOC₂H₅] to Ethyl (2S)-Hydroxypropionate

In a 2 ml microanalysis bottle equipped with a magnetic stirrer, 10 mg of 5% Pt/Al₂O₃ (catalyst JMC 94, batch 14017/01, pretreated under hydrrogen at 400° C. for 2 hours) are initially charged and admixed with 1 mg of modifier. 100 microlitres of ethyl pyruvate dissolved in 1 ml of solvent are then added, and the microanalysis bottle is then placed in a 50 ml pressure autoclave together with three further microanalysis bottles. The autoclave is purged three times with argon and three times with hydrogen and then 60 bar of hydrogen are injected in. The reactions are started by switching on the magnetic stirrer and carried out at room temperature. After 60 to 70 minutes, the pressure is dissipated, and the autoclave is purged three times with argon and opened. The catalysts are filtered off and the reaction mixture is analysed. The results are reported in Table 1.

TABLE 1 [Abbreviations: AcOH is acetic acid] Example No. Modifier Solvent ee (%) Conversion (%) B1 MeO-Cn AcOH 85 100 B2 MeO-Qd AcOH 90 100 B3 iso-Cn AcOH 88 100 B4 iso-Qd AcOH 84 100 B5 EXN-1 AcOH 88 100 B6 rubanol AcOH 82 100 Comparative Cn AcOH 88 >99 Comparative HCn AcOH 88 >99 B7 MeO-Cn toluene 31 100 B8 MeO-Qd toluene 53 100 B9 iso-Cn toluene 74 100 B10 iso-Qd toluene 61 100 B11 EXN-1 toluene 69 100 B12 rubanol toluene 67 100 Comparative Cn toluene 68 >99 Comparative HCn toluene 65 >99

EXAMPLES B13-B24 Hydrogenation of Methyl Phenylketoacetate

The procedure of Example B1 is followed using methyl phenylketoacetate. The results are reported in Table 2.

TABLE 2 Example No. Modifier Solvent ee (%) Conversion (%) 13 MeO-Cn AcOH 18 100 B14 MeO-Qd AcOH 15 100 B15 iso-Cn AcOH 75 100 B16 iso-Qd AcOH 15 100 B17 EXN-1 AcOH 53 100 B18 rubanol AcOH 37 100 Comparative HCn AcOH 51 100 B19 MeO-Cn toluene 31 100 B20 MeO-Qd toluene 8 100 B21 iso-Cn toluene 80 100 B22 iso-Qd toluene 66 100 B23 EXN-1 toluene 70 100 B24 rubanol toluene 90 100 Comparative HCn toluene 78 100

EXAMPLES B25-B36 Hydrogenation of Methylglyoxal 1,1-dimethyl Acetal

The procedure of Example B1 is followed using methylglyoxal 1,1-dimethyl acetal. The results are reported in Table 3.

TABLE 3 Example No. Modifier Solvent ee (%) Conversion (%) B25 MeO-Cn AcOH 93 100 B26 MeO-Qd AcOH 92 100 B27 iso-Cn AcOH 82 97 B28 iso-Qd AcOH 81 100 B29 EXN-1 AcOH 84 100 B30 rubanol AcOH 69 100 Comparative Cn AcOH 76 94 Comparative HCn AcOH 78 96 B31 MeO-Cn toluene 19 71 B32 MeO-Qd toluene 29 77 B33 iso-Cn toluene 42 79 B34 iso-Qd toluene 26 55 B35 EXN-1 toluene 19 100 B36 rubanol toluene 72 100 Comparative Cn toluene 33 55 Comparative HCn toluene 20 65

EXAMPLES B37-B48 Hydrogenation of Ethyl 2,4-diketobutyrate to Ethyl (S)-4-keto-2-hydroxybutyrate

The procedure of Example B1 is followed using ethyl 2,4-diketobutyrate. The results are reported in Table 4.

TABLE 4 Example No. Modifier Solvent ee (%) Conversion (%) B37 MeO-Cn AcOH 59 100 B38 MeO-Qd AcOH 73 100 B39 iso-Cn AcOH 67 100 B40 iso-Qd AcOH 60 100 B41 EXN-1 AcOH 74 98 B42 rubanol AcOH 61 96 Comparative HCn AcOH 64 100 B43 MeO-Cn toluene 43 100 B44 MeO-Qd toluene 31 90 B45 iso-Cn toluene 70 100 B48 iso-Qd toluene 66 93 B47 EXN-1 toluene 66 100 B48 rubanol toluene 35 100 Comparative HCn toluene 64 100

EXAMPLES B49-B60 Hydrogenation of Ethyl 2,4-dioxo-4-phenylbutyrate to Ethyl (S)-4-keto-4-phenyl-2-hydroxybutyrate

The procedure of Example B1 is followed using ethyl 2,4-dioxo-4-phenylbutyrate. The results are reported in Table 5.

TABLE 5 Example No. Modifier Solvent ee (%) Conversion (%) B49 MeO-Cn AcOH 62 100 B50 MeO-Qd AcOH 61 100 B51 iso-Cn AcOH 62 97 B52 iso-Qd AcOH 61 100 B53 EXN-1 AcOH 74 100 B54 rubanol AcOH 61 100 Comparative HCn AcOH 64 100 B55 MeO-Cn toluene 18 100 B56 MeO-Qd toluene 4 100 B57 iso-Cn toluene 71 99 B58 iso-Qd toluene 4 10 B59 EXN-1 toluene 53 100 B60 rubanol toluene 62 100 Comparative HCn toluene 64 100

EXAMPLES B61-B71 Hydrogenation of Ethyl 4-phenyl-2-oxobutyrate

The procedure of Example B1 is followed using ethyl 4-phenyl-2-oxobutyrate. The results are reported in Table 6.

TABLE 6 Example No. Modifier Solvent ee (%) Conversion (%) B61 MeO-Cn AcOH 81 100 B62 MeO-Qd AcOH 82 100 B63 iso-Cn AcOH 81 100 B64 iso-Qd AcOH 76 100 B65 EXN-1 AcOH 86 100 B66 rubanol AcOH 78 100 Comparative HCn AcOH 78 100 B67 MeO-Cn toluene 16 100 B68 MeO-Qd toluene racemic 100 B68 iso-Cn toluene 66 100 B69 iso-Qd toluene 55 100 B70 EXN-1 toluene 46 100 B71 rubanol toluene 64 100 Comparative HCn toluene 57 >95

EXAMPLE B72 AND COMPARATIVE EXAMPLE Hydrogenation of Ethyl Pyruvate

5 mg of modifier are initially charged in a 50 ml pressure autoclave equipped with a magnetic stirrer and baffle. 50 mg of catalyst (JMC 94, batch 14017/01, pretreated under hydrogen at 400° C. for 2 h) are slurried in 2 ml of acetic acid and transferred to the autoclave. The substrate is dissolved in the rest of the solvent (total 20 ml) and likewise transferred to the autoclave. The autoclave is purged three times with argon and three times with hydrogen and then 60 bar of hydrogen are injected in. The reaction is started by switching on the magnetic stirrer. The temperature is kept constant at 25° C. with the aid of a cryostat. The pressure in the autoclave is kept constant during the reaction using a dome pressure regulator, and the hydrogen takeup in the reactor is measured by the pressure decrease in a reservoir. After the end of the reaction, the reactor is decompressed, and the autoclave is purged three times with argon and then opened. The catalyst is filtered off. The conversion is determined by gas chromatography. The results are reported in Table 7. HCn means 10,11-dihydrocinchonine.

TABLE 7 Time Conversion ee mmol of H₂/ Example Modifier (min) (%) (%) g of catalyst Comparative HCn 19 100 89 148 B72 A3 16 100 91 182

EXAMPLE B73 AND COMPARATIVE EXAMPLE Hydrogenation of Methylglyoxal 1,1-dimethyl Acetal

The procedure is the same as in Example B72. The conversion is determined by gas chromatography. The results are reported in Table 8. HCn means 10,11-dihydrocinchonine.

TABLE 8 Time Conversion ee mmol of H₂/ Example Modifier (min) (%) (%) g of catalyst Comparative HCn 120 51 71 24 B73 A3 120 61 79 35

EXAMPLES B74-B75 AND COMPARATIVE EXAMPLE Hydrogenation of Ethyl 4-phenyl-2,4-dioxobutyrate

The procedure is the same as in Example B72. The conversion is determined by gas chromatography. The results are reported in Table 9. HCn means 10,11-dihydrocinchonine

TABLE 9 Amount of Modifier Time Conversion ee mmol/ Example catalyst (amount in (min) (%) (%) g*min Comparative  10 mg HCn (2) 95 95 56 — B74 125 mg A3 (13) 60 94 78 6.72 B75  84 g A3 (8400) 90 99 79 2.7 

What is claimed is:
 1. A process for enantioselectively hydrogenating a prochiral ketone to an (S)-alcohol, which comprises hydrogenating the prochiral ketone in the presence of a platinum catalyst, a modifier and hydrogen, wherein the modifier is a compound of the formula XI with 8(R),9(S)-configuration

where R₉ is H or CH₃—CH═ and R₇ is H or methyl, and R₈ is C₆-C₁₄-aryl which is unsubstituted or substituted by C₁-C₄-alkyl or C₁-C₄-alkoxy, or R₈ is C₅-C₁₃-heteroaryl having a heteroatom selected from the group consisting of —N═, —O—, —S— and —N(C₁-C₄-alkyl)- which heteroaryl is unsubstituted or substituted by hydroxy, C₁-C₄-alkyl or C₁-C₄-alkoxy.
 2. A process according to claim 1, wherein the prochiral α-ketones are saturated or unsaturated, open-chain or cyclic compounds which contain 5 to 30 carbon atoms which are unsubstituted or substituted by radicals which are stable under the hydrogenation conditions, and the carbon chain is uninterrupted or interrupted by heteroatoms from the group of —O—, ═N— and —NR′— where R′ is H, C₁-C₈-alkyl, C₅-C₈-cycloalkyl, C₆-C₁₀-aryl or C₇-C₁₂-aralkyl.
 3. A process according to claim 2, wherein the prochiral ketone is selected from the group consisting of α-ketocarboxylic acids, α-ketocarboxylic esters, α-ketoethers, α-ketoacetals and α,β-diketones.
 4. A process according to claim 3, wherein the prochiral ketone corresponds to the formula, I, II, III, IV or V

where R₁, R₂, R₃ and R₆ are each independently a monovalent, saturated or unsaturated aliphatic radical having 1 to 12 carbon atoms, a saturated or unsaturated cycloaliphatic radical having 3 to 8 carbon atoms, a saturated or unsaturated heterocycloaliphatic radical having 3 to 8 ring members and one or two heteroatoms from the group of O, N and NR′, a saturated or unsaturated cycloaliphatic-aliphatic radical having 4 to 12 carbon atoms, a saturated or unsaturated heterocycloaliphatic-aliphatic radical having 3 to 12 carbon atoms and one or two heteroatoms from the group of O, N and NR′, an aromatic radical having 6 to 10 carbon atoms, a heteroaromatic radical having 4 to 9 carbon atoms and one or two heteroatoms from the group of O and N, an aromatic-aliphatic radical having 7 to 12 carbon atoms or a heteroaromatic-aliphatic radical having 5 to 11 carbon atoms and one or two heteroatoms from the group of O and N where R′ is H, C₁-C₈-alkyl, C₅- or C₆-cycloalkyl, C₆-C₁₀-aryl, or C₇-C₁₂-aryl, or R₁ and R₂ or R₁ and R₆ together are C₁-C₆-alkylene or C₃-C₈-1,2-cycloalkylene, or C₂-C₄-alkylene or C₃-C₈-cycloalkylene fused to 1,2-phenylene, and R₃ is as defined above, R₂ and R₃ together are C₁-C₆-alkylene, C₁-C₈-alkylidene, C₃-C₈-1,2-cycloalkylene, C₃-C₈-cycloalkylidene, benzylidene, 1,2-phenylene, 1,2-pyridinylene, 1,2-naphthylene, or C₃-C₄-alkylene or C₃-C₈-1,2-cycloalkylene fused to 1,2-cycloalkylene or 1,2-phenylene, and R₁ is as defined above, and R₁, R₂, R₃ and R₆ are each unsubstituted or substituted by one or more, identical or different radicals selected from the group of C₁-C₄-alkyl, C₂-C₄-alkenyl, C₁-C₄-alkoxy, C₁-C₄-haloalkyl, C₁-C₄-hydroxyalkyl, C₁-C₄-alkoxymethyl or -ethyl, C₁-C₄-haloalkoxy, cyclohexyl, cyclohexyloxy, cyclohexylmethyl, cyclohexylmethyloxy, phenyl, phenyloxy, benzyl, benzyloxy, phenylethyl, phenylethyloxy, halogen, —OH, —OR₄, —OC(O)R₄, —NH₂, —NHR₄, —NR₄R₅, —NH—C(O)—R₄, —NR₄—C(O)—R₄, —CO₂R₄, —CO₂—NH₂, —CO₂—NHR₄, —CO₂—NR₄R₅ where R₄ and R₅ are each independently C₁-C₄-alkyl, cyclohexyl, cyclohexylmethyl, phenyl or benzyl.
 5. A process according to claim 1, wherein R₈, as aryl and heteroaryl, is a monocyclic or fused polycyclic radical.
 6. A process according to claim 5, wherein the aryl and heteroaryl comprise rings having 5 or 6 ring members.
 7. A process according to claim 1, wherein the modifier is a compound of the formula XIa with 8(R),9(S)-configuration

where R₉ is H or CH₃—CH═ and R₇ is H or methyl, R₈ is a radical of the formula

and R₁₀ is H, OH or C₁-C₄-alkoxy.
 8. A process according to claim 1, wherein the platinum metal is used in an amount of 0.01 to 10% by weight, based on the prochiral ketone used.
 9. A process according to claim 1, wherein the modifier is used in an amount of 0.1 to 10 000% by weight, based on the platinum metal used.
 10. A process according to claim 1, wherein the hydrogenation is carried out under a hydrogen pressure of up to 200 bar.
 11. A process according to claim 1, wherein the hydrogenation is carried out at a reaction temperature of −50 to 100° C.
 12. A process according to claim 4, wherein R′ is C₁-C₄-alkyl.
 13. A process according to claim 4, wherein R′ is phenyl or naphthyl.
 14. A process according to claim 4, wherein R′ is phenylmethyl or phenylethyl. 